The invention relates generally to dye transfer printers such as thermal printers, and in particular to the problem of crease formation in the dye transfer area of a donor web used in the printer. Crease formation in the dye transfer area can result in an undesirable line artifact being printed on a dye receiver.
A typical multi-color donor web that is used in a thermal printer is substantially thin and has a repeating series of three different color sections or patches such as a yellow color section, a magenta color section and a cyan color section. Also, there may be a transparent laminating section after the cyan color section.
Each color section of the donor web consists of a dye transfer area that is used for dye transfer printing and pair of longitudinal edge areas alongside the transfer area which are not used for printing. The dye transfer area is about 95% of the web width and the two edge areas are each about 2.5% of the web width.
To make a print, the various color dyes in the dye transfer areas of a single series of yellow, magenta and cyan color sections on a donor web are successively heat-transferred by a print head onto a dye receiver such as paper or transparency sheet or roll. The dye transfer from each transfer area to the dye receiver is done line-by-line widthwise across the transfer area via a bead of selectively heated resistive elements on the print head. The print head makes line contact across the entire width of the color section, but it only heats the dye transfer area, i.e. it does not heat the two edge areas alongside the dye transfer area.
As each color section is used for dye transfer at the print head, the donor web is subjected to a longitudinal tension between a donor supply spool and a donor take-up spool which are rearward and forward of the print head, and particularly at a fixed web guide between the print head and the donor take-up spool. The longitudinal tension, coupled with the heat from the print head, causes a used color section to be stretched lengthwise at least from the print head to the donor take-up spool. Since the dye transfer area in a used color section has been heated by the print head, but the two edge areas alongside the transfer area have not been heated, the transfer area tends to be stretched more than the edge areas. As a result, the transfer area becomes thinner than the two edge areas and develops a wave-like or ripple distortion widthwise between the edge areas.
After the last line is transferred from a dye transfer area to a dye receiver, and as the used color section is advanced forward from the print head and onto the donor take-up spool, the wave-like or ripple distortion in the transfer area causes one or more creases to form at least in a short trailing or rear end portion of the transfer area that has not been used for dye transfer. The creases tend to spread rearward from the trailing or rear end portion of the used transfer area into a leading or front end portion of an unused transfer area in the next (fresh) color section being advanced to the print head. The creases appear to be created because of the difference in thickness between the used transfer area and the edge areas as they are wound under tension from the print head and onto the donor take-up spool.
A problem that can result is that a crease in the leading or front end portion of the unused transfer area of the next (fresh) color section will cause an undesirable line artifact to be printed on a leading or front end portion of the dye receiver when the print head is applied to the crease. The line artifact printed on the receiver is about 0.5 inches in length.
The question presented therefore is how to solve the problem of the creases being created in the unused transfer area of each fresh color section so that no line artifacts are printed on the dye receiver.
A dye transfer printer in which a dye donor web is capable of developing a crease-causing wave-like or ripple distortion across the donor web when the donor web is subjected to a longitudinal tension as it is advanced from a print head, over a web guide, and onto a web take-up spool, is characterized in that:
the web guide is positioned to extend across the donor web and is adapted to be bowed to effect a curvature across the donor web in proportion to the longitudinal web tension in order to urge the donor web to spread substantially widthwise to reduce the likelihood of the wave-like or ripple distortion developing across the donor web.
If the wave-like or ripple distortion is prevented from developing across the donor web, it is unlikely that any of the creases will be created in the unused transfer area of each fresh color section. Thus, no line artifacts can be printed on the dye receiver.